1. Field of the Invention
This invention relates to optical waveguide fibers and, in particular, to an improved protective tube for such fibers.
2. Description of the Prior Art
As is well known in the art, optical waveguide fibers are mechanically fragile and exhibit degraded light transmission when bent ("bend losses"). Accordingly, in most applications, fibers are housed in protective coverings. These coverings are generally referred to as "tubes" when individual fibers are being protected or as "cables" when multiple fibers have been grouped together and protected as a unit. Also, two general packaging arrangements for optical waveguide fibers have been recognized in the art--those which include a tight buffer layer which is in direct contact with the fibers, and those in which the fiber is loose within the covering. The present invention is concerned with protective coverings of the tube type, i.e., protection for a single fiber, wherein the fiber is loose within the tube, that is, the fiber can be pulled out of a reasonable length of tube, e.g., three feet (hereinafter referred to as a "loose tube" construction).
The design of protective coverings and, in particular, loose tube coverings has proved to be a difficult problem since the coverings must satisfy a variety of stringent and, in many cases, conflicting requirements.
Thus, the coverings need to be strong enough to protect the fiber from substantial tensile (longitudinal) loads, and yet flexible enough to bend without kinking since kinking results in high bend losses. Moreover, in many cases, the coverings must be tough enough to withstand prolonged exposure to harsh environmental conditions, again without losing their flexibility. The coverings also must be easily strippable from the fiber both in a manufacturing setting and in the field.
In addition to these requirements, the protective covering should also be as small as possible, i.e., the finished covering/fiber structure should have a small an outside diameter (OD) as possible so as to minimize space consumption in plenums, junction boxes, and the like. Commercially available, high strength protective coverings which have included reinforcing strength members, e.g., aramid fibers, have had OD's greater than about 2,000 microns, with 2,900 microns being a typical dimension. Non-reinforced protective tubes having OD's on the order of 1,000 microns have been prepared (see discussion of the SILEC protective tubes below). However, prior to the present invention, the 1,000 micron level has not been achieved in a reinforced, loose tube construction.
For some applications, the requirements for the coverings are even more stringent. In particular, optical waveguide couplers, i.e., devices in which at least a portion of the light propagating in one fiber is coupled to one or more other fibers, add the further requirement that the protective covering must have a high thermal stability. This requirement is particularly acute in the case of achromatic couplers which operate at more than one frequency, e.g., 1300 and 1550 nanometers, since longer wavelength signals tend to be more sensitive to bend losses.
The origin of the thermal matching requirement can be seen in FIG. 1 which illustrates a typical coupler construction. A general discussions of couplers can be found in the following U.S. patents, the relevant portions of which are incorporated herein by reference: Keck, U.S. Pat. No. 4,704,151; Dohan et al., U.S. Pat. No. 4,765,702; Miller et al., U.S. Pat. No. 4,902,324; Beguin, U.S. Pat. No. 4,933,262; and Dannoux et al., U.S. Pat. No. 4,943,130.
As shown in FIG. 1, coupler 11 includes coupler body 15 and a plurality of "pigtails" 19 which extend from the coupler body. Each pigtail includes a connector 17, an optical waveguide fiber 35 (FIG. 2) which connects the body to the connector, and a protective tube 13 which loosely surrounds the optical waveguide fiber and is connected to the body and the connector. The ends of both the optical waveguide fiber and the protective tube 13 are fixed at both the coupler body and the connector. Accordingly, differences in the thermal expansion coefficients of the fiber and the tube over the operating range of the coupler, e.g., from -40.degree. C. to +85.degree. C., will manifest themselves as relative changes in the lengths of the fiber and the tube.
In particular, when the tube becomes shorter than the fiber, the fiber will bend resulting in bending losses which can be as large as 10-20 dB depending upon the magnitudes of the temperature change and the expansion coefficient mismatch. On the other hand, when the tube becomes longer than the fiber, actual fractures of the fiber have been observed in extreme cases. Since optical waveguide fibers are primarily composed of silica and since silica has a low thermal expansion coefficient, i.e., on the order of 10.sup.-7 cm/cm/.degree. C., the thermal matching requirement basically means that the protective tube must have limited expansion and contraction over the operating range of the coupler, a difficult requirement to meet since plastic materials typically have relatively high expansion coefficients.
Because of the difficulty in satisfying all of the foregoing requirements simultaneously, a variety of protective tube and cable constructions have been proposed in the art. Examples of such constructions are as follows.
Protective tubes for use in couplers have been sold by the Societe Industrielle de Liaisons Electriques, Paris, France, under the trademark SILEC and the product designation CDR 2. See SILEC's product catalog entitled "Flexible Protective Tubes for Optical Fibers," Notice No. SCFO-06, 1987. These tubes are composed of a polypropylene inner layer surrounded by a polyethylene outer layer and are said to have a reduced expansion coefficient. The tubes have an outside diameter of 1,100 microns. In practice, the SILEC tubes have been found to be relatively stiff and to contract upon heating. Comparative data for the SILEC tubes and the tubes of the present invention is presented below.
Tubes and cables having various constructions have been sold by Siecor Corporation, Hickory, N.C. See The Fiber Optic Catalog--1988-1989, pages 1.2, 1.3, and 1.20, 1988. In particular, Siecor has sold "Fan-Out Tubing" which has an OD of 2,900 microns and is composed of a PVC outer jacket, a layer of aramid fiber, and a fluoropolymer inner tube. The inner tube surrounds an optical fiber in a loose tube construction. Siecor has also sold "Interconnection Cables" comprising a PVC outer jacket and a thermoplastic layer in direct contact with the optical fiber, i.e., a tight buffered construction. The cable has an outside diameter of 2,900 microns. The thermoplastic layer can be a polyester elastomer such as DuPont's HYTREL brand elastomer. The PVC jacket and the thermoplastic layer are not in contact but are separated by a layer of stranded aramid fiber. This layer, plus the tight-buffered construction, makes the Interconnection Cable more difficult to strip than the protective tube of the present invention.
Fuse et al., U.S. Pat. No. 4,629,286, discloses a tight buffered construction in which buffer layer 3 is in contact with resin layer 4 which, in turn, is in contact with reinforced layer 5. An ultrafine gap can be left between the buffer layer and the resin layer. Reinforced layer 5 comprises a resin matrix in which are embedded strength members such as glass fiber, carbon fiber, or aramid fiber. The resin matrix can be composed of a heat-curable, unsaturated polyester, an epoxy, a silicone or vinyl ester, or a heat-curable polyamide. The resin layer 4 can be composed of the same material as the resin matrix of layer 5 or, preferably, is composed of a thermosetting resin such as a polyester resin or a polyamide resin, or a urethane or epoxy acrylic compound. The completed fiber/tube combination has an outside diameter of between 950 and 1,000 microns.
Johnson et al., U.S. Pat. No. 4,723,831, discloses an optical fiber cable which includes core wrap 12, which is composed of woven fiber glass which has been impregnated with polytetrafluoroethylene, and jacket 15, which is composed of polyvinyl chloride (PVC). Embedded in the PVC jacket are three equally-spaced strength members 16 which are preferably composed of glass fibers. Alternatively, the strength members can be composed of KEVLAR brand aramid fiber. The strength members are impregnated with a material such as a urethane, an acrylic acid or acrylate-based material, an epoxy, a polyester, or a polyvinyl chloride or other vinyl-based material so as to produce a strong coupling between the strength members and jacket 15.
Arroyo, U.S. Pat. No. 4,730,894, discloses an optical fiber cable which includes strength members 86 which are adhesively bonded to a carrier tape 82 made of MYLAR brand polyester film. The preferred strength members are glass rods having a diameter of 0.035 inches (890 microns) held together in a polyurethane matrix. KEVLAR brand aramid yarn is also mentioned as a possible strength member. All but a small portion of the circumference of each strength member is embedded in outer jacket 54 which is preferably composed of high density polyethylene and has a wall thickness of 0.050 inches (1270 microns). The cable includes an inner tube 28 also of high density polyethylene which has a wall thickness of 0.030 inches (760 microns). The overall outside diameter of the cable is thus at least 2,000 microns.
Ramsay et al., U.S. Pat. No. 4,756,600, discloses an optical fiber cable of the tight buffered type having coating layer 4 which can comprise a HYTREL brand polyester elastomer, strength layer 32 composed of two layers of KEVLAR brand aramid fiber held in place by polyester wrap 33, and outer layer 31 also composed of HYTREL. The overall cable has an outside diameter of 3,000 microns.
Taylor et al., U.S. Pat. No. 4,776,910, discloses an optical fiber cable which employs aromatic polyamide strength members both in its outer sheath 8 and along the center of the bore of internal sheath 5. Sheaths 8 and 5 are both composed of polyethylene.
Calzolari et al., U.S. Pat. No. 4,932,746, discloses an optical fiber cable having: (a) a central strength member 1, which can be made of an aromatic polyamide, (b) a plurality of small tubes 3 each of which carries an optical fiber, and (c) a core 2 in which the small tubes are embedded. The patent states that the small tubes and the core should be made of materials which do not bond or link even at their softening temperature. Among the materials which can be used to make the core/tube are polyamides, polybutene terephthalate, low density polyethylene, polypropylene, and polyurethanes. See also Calzolari et al., U.S. Pat. No. 4,902,096.
U.K. Patent application No. 2,185,828 discloses an optical fiber cable which includes a reinforcing layer 25 which comprises KEVLAR brand fibers embedded in a plastic material which can be a thermoplastic material, such as polypropylene or nylon, or a thermosetting material, such as, a polyester or epoxy resin or a polyurethane. Surrounding the reinforcing layer is a protective sheath 27 composed of polyethylene.